In the fabrication of flip-chip microelectronic assemblies, it is conventional to use a capillary flow underfill process which involves first making a circuit board by applying a fluxing agent onto a substrate, following with a placement of the flip-chips having solder balls on the substrate, and subjecting the assembly to a first heating cycle to melt the solder balls, so as to create operable interconnections between the substrate and the electronic elements in the flip-chips electronic elements. A circuit board having flip-chips connected to the substrate is thereby formed. An underfill or adhesive material is introduced only after the interconnections (or after the circuit board) has been fabricated. This typically involves a further step of applying the adhesive material along the circuit board and allowing the adhesive material to flow into the spaces between the solder interconnections by capillary forces. The circuit board with the adhesive material disposed thereon is then subjected to a second heating cycle to as to cure the adhesive material, i.e., to cause cross-linked network bonding within the adhesive material domain, but not to cause melting of the solder lest the interconnections formed during the first heating cycle are damaged. As can be appreciated, in one process of making a flip-chip assembly, two heating cycles are required. Since each heating cycle requires time for the temperature to be elevated to a desired level and time for cooling, there is a strong need to increase the efficiency of the flip-chip assembly process in terms of cutting down the production time and reducing the energy consumed.
As a result, there have been efforts to develop one-cycle heating processes, such as the hybrid no-flow process or the no-flow underfill process.
Michael Colella and Daniel Baldwin (Proceedings of the Electronic Components and Technology, IEEE conference 2004) disclosed no-flow processes involving one-cycle heating in which a reflow encapsulant material that exhibits fluxing underfill material properties is introduced to the substrate before placement of the flip-chips on the substrate. It is observed that in such processes, correct placement of the solder balls relative to the contact pads or terminals on the substrate is a problem, and proper connections between the substrate and the flip-chips are not formed.
The U.S. Pat. No. 6,943,058 B2 discloses another approach of achieving one-cycle heating by proposing a no-flow underfill material that initially comprises a dielectric polymer material and a precursor capable of forming an inorganic filler. The flip-chip or component is placed on the substrate after the underfill material is dispensed over terminals on a substrate (Col 3 ln 1-8). Similar issues such as incorrect placement of the flip-chips and a lack of connection between the flip chips and the substrate are observed.
There remains therefore a need for an improved method of forming flip-chip assemblies with improved production yields, i.e., less likelihood of inadequate interconnection between the flip-chips and the substrate leading to failure of the microelectronic product as a whole. At the same time, there is a pressing need for more efficient processes that require less time for the completion of the flip-chip assembly. Further, consistent with the spirit of environment-friendly manufacturing, there is a need to reduce the amount of energy consumed in the production of flip-chip assemblies.